THE AMINO ACID SEQUENCES OF CYTOCHROMES C-553 AND C-550 (CYANOBACTERIA, PHOTOSYNTHESIS, ELECTRON TRANSPORT)
Abstract
The amino acid sequences of cytochromes C(,553) from two divergent photosynthetic organisms are reported. Aphanizomenon flos-aquae cytochrome C(,553) is the first basic (pI = 9.3) photosynthetic cytochrome to be reported. The acidic cytochrome from Porphyridium cruentum is the second red algal cytochrome to be sequenced. Amino acid sequence comparisons with other sequenced cyanobacterial cytochromes indicates that a region between residues 62 and 72 varies in charge in a way that parallels the variation in pI of the cyanobacterial cytochromes. This region is shown to correspond to the region on respiratory cytochromes which interacts with the cytochrome oxidase. The acidic cytochrome C(,553) from Spirulina maxima was chemically modified with ethylenediamine and a water soluble carbodiimide in an effort to convert carboxyl groups on the protein into amines. Under the reaction conditions used, all or nearly all carboxyl groups were converted into basic groups, as indicated by the high pI (>10) of the modified protein, without altering its three-dimensional integrity. The partial amino acid sequence of the low redox potential cytochrome C(,550) from Microcystis aeruginosa is reported. The cytochrome is unique in that it is approximately 140 amino acids long, longer than most cytochromes c hitherto described, and has a 37 residue N-terminal sequence preceding the heme. Homology with other c-type cytochromes was limited to the immediate vicinity of the heme binding site. The heme could not be removed from the protein using all but the strongest oxidizing conditions, suggesting a unique attachment of the heme to the protein. The absence of a methionine as the sixth ligand to heme-iron atom is also suggested.
Degree
Ph.D.
Subject Area
Biochemistry
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